Unearthed or IT systems are problematic for RCDs due to lack of an earth-path and the 1stand 2ndfault scenario. Although widely regarded as the safest earthing system, they are not widely used in information technology applications as a result.
When the RVD is operating in an IT or floating environment, the RVD essentially becomes the center tap/reference point of the transformer, being held at zero volts. The RVD is detecting a voltage rise from an insulation breakdown on either L1 or L2 onto the frame being protected. When a voltage above the trigger level of 43V is detected, the RVD causes a 33mA imbalance across the RCD which isolates the circuit. When this occurs, 8mA of fault current flows with isolation occurring in under 20mSec.